And Simsion couldn't be more pleased that his book has been so widely embraced. The response has been so far beyond what anybody could expect from a debut that he is, in many ways, still reeling.

His hero, Don Tillman – a man with Asperger's Syndrome who sets out to find the perfect woman via a spreadsheet – has almost taken on a life beyond the page.

"I think what I love about The Rosie Project is that it is a book that is appreciated and discussed at all levels," he says, a week before the publication of the book's much anticipated follow-up, The Rosie Effect.

"I don't want to sound like a snob here, but you can be someone who reads only romance books – your reading is genuinely Mills and Boon and almost nothing else – and somebody says, 'Well, this is a romance with a twist', and you will have a reaction, a response to it."

The response is generally good. Don may be intelligent and in rude good health, and, in Rosie's eyes at least, have a great physique, but he's no Mr Darcy. And yet, men seem to relate to him – his tunnel vision, his initial unwillingness to compromise on his version of perfection – while women fall in love with him.

And, at the other end of the reader spectrum, via non-literary types who like a good yarn, serious readers with dedicated book groups and literary reviewers who have, by and large taken a liking to this off-beat romance, is Bill Gates.

Yes, the computer programmer, philanthropist and billionaire American business magnate. Simsion says he was as surprised as anyone when he learned that Gates and his wife Melinda wanted to meet him two weeks ago. On holidays in England at the time with his wife, Simsion made the snap decision to reroute their trip and return via Seattle to chat to the Gates, because who in their right minds would pass up such an opportunity?

"We basically just had a chat about The Rosie Project and The Rosie Effect – Simon & Schuster managed to get them an advanced reader copy only a few days before the meeting but they'd both read it," he says.

"We talked about both books, what they'd taken from them, all those sorts of things, and what was interesting, I guess, is that Melinda found it absolutely laugh-out-loud funny, and Bill also found it funny, but he also found it profound – that was his word."

The Gates were equally impressed with the second instalment, which sees Don and Rosie spending their first year of marriage in New York. It's a lot longer, and markedly more complex than the first, veering into some more serious territory as we learn more about how Don sees the world and why. When he is once more pushed right out to the edge of what he can cope with when Rosie announces she's pregnant, the response is visceral, moving and predictably hilarious, as the couple negotiate the adjustment in their own, parallel worlds.

Simsion admits he never planned to write a sequel once the first Rosie was wrapped up. He was onto other projects, and it wasn't until he was out celebrating the pregnancy of a friend, and, as a father of two, giving the father his "expectant dad" talk, that he realised the potential of another instalment.

"In the end, the first book, Don was taken right out to the edge, everything was on the line for him – does he have a connection with the other person, does he not? And there was no point writing another book unless I took Don out to the edge again," he says.

"I thought OK, Don's moved to New York, what would hit him with the maximum possible force? And we take Don to another place. But pregnancy, it's one of those topics that men don't dare touch, really, and it's such a staple of chick-lit and I was very conscious that I was not going to write this from Rosie's perspective. We don't know what happens behind closed doors with Rosie, or what she's saying to her best friend or anything like that, we only know what the man sees, and I can write authentically about that."

It was, he says, much easier to write the second novel than the first – perhaps to do with his analytical background.

"I'm a pretty reflective guy. I've come out of a management technical background, and with those backgrounds, if something doesn't work you don't do it again, and you learn from it. If something works you do it again," he says.

It's not hard to guess why the Rosie books would resonate with someone like Gates, but for Simsion, it's further affirmation that the character he created has some level of authenticity.

"People are finding it authentic. I've had my first feedback from the Asperger's community about The Rosie Effect from someone who's married to a man with Asperger's, saying, 'I didn't think I'd like the book more than the first one, but this actually goes deeper', so it's so nice to know I haven't missed the mark and got it wrong the second time," he says.

The book's success, and its continuing presence on The New York Times bestseller list, means a film adaptation is only a matter of time. In fact, the book began life as a screenplay, and Simsion has already sold the rights to Sony films. Names such as Steve Carell, Jon Hamm, Hugh Jackman and Benedict Cumberbatch are being bandied about, as are Jennifer Lawrence, Carey Mulligan and Emma Stone.

Simsion freely admits to having put a lot of idle though into a potential cast for the film, but hopes only that the choice of actor to play Don is innovative.

"It's one of those games! Frankly I get asked the question so often, and of course I'm just like anybody else, you fantasise – who would it be?"

Despite his particular quirks as a romantic lead, Simsion says there is something universal about Don, or at least recognisable.

"I didn't write him from a textbook, but I was talking to Tony Attwood, who's a guru on Asperger's Syndrome, and I was giving him my usual thing – I hadn't read any textbooks, I've just based him on people I know, I can't say if he's got Asperger's, I'm not a psychologist, and Tony said, 'Listen, I am, and Don Tillman has Asperger's'. So it's official, if you like," he says.

"But I think Asperger's is probably genetic…[and] I think that then pops up in every society. So in that sense, in that very simplistic sort of sense, he's a universal sort of character. We've sold the rights to the book in 42 countries now, Taiwan and Albania and all sorts of places, Vietnam, Indonesia and Russia, so clearly when they read that book they see something in there which they're able to recognise. If you couldn't recognise the Don character, I don't think it would be too exciting."

Although Simsion's life on the surface hasn't changed as profoundly since The Rosie Project as you might think – he already had a successful career in information technology, travelled frequently and spoke at seminars across the world – writing is, in every way, a second career.

"Something hit me the other day, that my second career became a genuine second career at the point that we sold the overseas rights for The Rosie Project," he says.

"At that point, I had an income that meant that I could give up my other career, so it moved from being a part-time background thing to being the primary career, and I did that at the same age as my father retired – 57. It's a scary thought."

Graeme Simsion will be speaking about The Rosie Effect and signing books on Tuesday, October 14, at the Australian National University, Manning Clark Theatre 2, at 6pm. Entry is free but bookings essential, at events@anu.edu.au or 6125 8415. The Rosie Effect is published by Text.